1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus. More particularly, the present invention relates to a developing device of the type including a toner carrier for conveying toner deposited thereon to a developing region where it faces an image carrier, and a toner supply member for conveying a two-ingredient type developer, which consists of the toner and magnetic particles, to a toner supplying region where it faces the toner carrier and feeds only the toner to the toner carrier, an image forming apparatus including the same, and an image forming process unit.
2. Discussion of the Background
Generally, a developing device for an image forming apparatus uses either one of a single-ingredient type developer, i.e., toner and a two-ingredient type toner that is a mixture of toner and magnetic carriers. In any case, the developing device develops a latent image formed on an image carrier with the toner. The developing device using a single-ingredient type toner is simple in construction and small size. Further, this type of developing device, which does not use magnetic particles, is free from so-called magnet brush marks particular to the developing device using a two-ingredient type developer. The magnet brush marks refer to an occurrence that a latent image is not faithfully developed due to the influence of a magnet brush, which is formed by the developer, contacting the image carrier. In addition, the developing device using a single-ingredient type developer forms a thinner developer later than the developing device using a two-ingredient type developer and suffers from a minimum of edge effect. For these reasons, this type of developing device can desirably reproduce high-definition images. Particularly, the developing device using only nonmagnetic toner is highly adaptive to color printing and can produce high-definition color images.
The developing device using a single-ingredient type developer charges the toner deposited on the toner carrier by friction with a blade, toner supply roller or similar contact member contacting the toner carrier. With this charging scheme, however, it is difficult to promote rapid control over the charge of toner and the enhancement of durability. Further, the contact member, pressing the toner on the toner carrier, brings about stress in the toner and therefore toner filming. In addition, the contact member is likely to cause an agent covering the individual toner particle to penetrate into the particle, degrading image quality. Moreover, the toner carrier and contact member wear due to friction acting therebetween and vary the developing characteristic due to aging. To solve such problems, it is preferable to supply toner charged to preselected polarity to the toner carrier without using the blade, toner supply roller or similar contact member.
In light of the above, Japanese Patent Laid-Open Publication Nos. 56-40862 and 59-172662, for example, each disclose a developing device including a toner supply member on which a two-ingredient type developer forms a magnet brush. In this type of developing device, only toner is fed from the magnet brush to a toner carrier. The toner supply member is implemented by a magnet roller or a magnet brush forming body. Toner in the magnet brush is charged to preselected polarity by friction acting between it and magnetic particles. Only the toner particles charged to the preselected polarity is separated from the magnet brush and transferred to the toner carrier, which is a developing roller or a toner layer support.
However, the problem with the developing device of the type transferring only toner from the magnet brush formed on the toner supply member is that when it develops a latent image formed on the image carrier, background contamination and short image density occur and degrade image quality. We experimentally found that the amount of charge deposited on the toner on the toner carrier was not distributed in an expected manner, resulting in background contamination and short image density. For example, when the number of toner particles charged to polarity opposite to expected polarity increases, they contaminate the background. Also, when the number of excessively charged toner particles increases, toner particles available for development decreases, resulting in short image density.
An arrangement may be made such that the charge distribution of toner on the toner carrier and the charge distribution on the toner supply member are the same with each other. However, the charge distribution of toner on the toner supply member is apt to differ from expected one due to the frictional charging characteristic required of the magnet brush formed on the toner supply member. In such a case, the charge distribution on the toner carrier, which received toner from the toner supply member, is also deviated from expected one.
The developing device of the type transferring only toner from the magnet brush on the toner supply member to the toner carrier may be used to develop a latent image formed on the image carrier by a so-called bilevel process. Today, a bilevel process is extensively used as an image forming process dealing with digital image data. Specifically, the bilevel process controls the individual pixel on the basis of whether or not a dot of preselected density and preselected size exists. The bilevel process renders tonality in terms of the density of, among pixels forming an image, pixels on which toner is to deposit for a unit area.
However, when the developing device of the type described develops a latent image formed on the image carrier by the bilevel process, background contamination and short image density also occur, depending on various conditions. For example, assume that the surface potential of the image carrier or the amount of charge or similar developing characteristic of the developer varies due to the varying environment or aging. Then, unnecessary toner deposits around dots, i.e., pixels on which toner is expected to deposit, contaminating the background or making the density of the dots short.
It is therefore an object of the present invention to provide a developing device capable of forming a high-quality toner image free from background contamination and short density without resorting to a contact member that charges toner on a toner carrier by friction, an image forming apparatus including the same, and an image forming process unit.
In accordance with the present invention, a developing device includes a toner carrier for carrying toner on its surface and conveying it to a developing region where the toner carrier faces an image carrier. A toner supply member carries thereon a two-ingredient type developer, which consists of the toner and magnetic particles, and conveys it to a toner supplying region where the toner supply member faces the toner carrier. Only the toner is fed from the toner supply member to the toner carrier in the toner supplying region. A number distribution, which is the distribution of the number of toner particles for an amount of charge deposited on the toner particles, differs from the toner particles conveyed to the developing region by the toner carrier to the toner particles contained in the developer that is deposited on the toner supply member.
Also, in accordance with the present invention, in an image forming apparatus including an image carrier, a latent image forming device for forming a latent image on the image carrier, a developing device for developing the latent image to thereby produce a corresponding toner image, and an image transferring device for transferring the toner image to a recording medium, the developing device includes a toner carrier for carrying toner on its surface and conveying it to a developing region where the toner carrier faces an image carrier. A toner supply member carries thereon a two-ingredient type developer, which consists of the toner and magnetic particles, and conveys it to a toner supplying region where the toner supply member faces the toner carrier. Only the toner is fed from the toner supply member to the toner carrier in the toner supplying region. A number distribution, which is the distribution of the number of toner particles for an amount of charge deposited on the toner particles, differs from the toner particles conveyed to the developing region by the toner carrier to the toner particles contained in the developer that is deposited on the toner supply member.
Further, in accordance with the present invention, in an image forming process unit including an image carrier, a charger for uniformly charging the surface of the image carrier, a developing device for developing the latent image to thereby produce a corresponding toner image, and a cleaning device for cleaning the surface of the image carrier that are constructed into a unit removable from an image forming apparatus, the developing device includes a toner carrier for carrying toner on its surface and conveying it to a developing region where the toner carrier faces an image carrier. A toner supply member carries thereon a two-ingredient type developer, which consists of the toner and magnetic particles, and conveys it to a toner supplying region where the toner supply member faces the toner carrier. Only the toner is fed from the toner supply member to the toner carrier in the toner supplying region. A number distribution, which is the distribution of the number of toner particles for an amount of charge deposited on the toner particles, differs from the toner particles conveyed to the developing region by the toner carrier to the toner particles contained in the developer that is deposited on the toner supply member.